1. Field of Invention
The present invention is related in general to systems and methods involving the use of magnetic field sensors. In particular, the invention is related to systems, methods, and apparata involving sensors and circuits that provide measurements of torque-induced magnetic fields, as well as magnetic field noise.
2. Description of the Related Art
In the control of systems having rotating drive shafts, torque and speed are fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable, and inexpensive manner has long been a primary objective of such control system designs.
Previously, torque measurement was accomplished using contact-type sensors directly attached to a shaft. One such sensor is a “strain gauge” type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by torque-induced strain is measured by a bridge circuit or other well known means. Contact-type sensors, however, are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are expensive and are thus commercially impractical for competitive use in many applications, such as automotive steering or transmission systems, for which torque sensors are sought.
Subsequently, non-contact torque sensors of the magnetostrictive type were developed for use with rotating shafts. For example, U.S. Pat. No. 4,896,544 to Garshelis, which is incorporated herein by reference, describes a sensor comprising a torque-carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically-directed residual stress-induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torqued member, differences in the response of the two bands to forces applied to the torque-carrying member. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the external magnetic fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used can create practical problems in applications where space is at a premium. Also, such sensors may be impractically expensive for use on highly cost-competitive devices, such as in automotive applications.
Torque transducers based on measuring the field arising from the torque induced tilting of initially circumferential remanant magnetizations have been developed which, preferably, utilize a thin wall ring (“collar”) serving as the field generating element. See, for example, U.S. Pat. Nos. 5,351,555 and 5,520,059 to Garshelis, which are incorporated herein by reference. Tensile “hoop” stress in the ring, associated with the means of its attachment to the shaft carrying the torque being measured establishes a dominant, circumferentially directed, uniaxial anisotropy. Upon the application of torsional stress to the shaft, the magnetization reorients and becomes increasingly helical as torsional stress increases. The helical magnetization resulting from torsion has both a radial component and an axial component, the magnitude of the axial component depending entirely on the degree of torsion. One or more magnetic field vector sensors may be used to sense the magnitude and polarity of the field arising, as a result of the applied torque, in the space above the magnetically conditioned regions on a shaft, and provide a signal output reflecting the magnitude and direction of the torque. Inasmuch as the peak allowable torque in a ring sensor is limited by slippage at the ring/shaft interface, concerns have been expressed regarding distortion arising from slippage at the ring/shaft interface under conditions of torque overload. This, together with the need for multiple parts of different materials to minimize the adverse effects of parasitic fields, has encouraged the investigation of alternative constructions.
Magnetoelastic torque transducers have been developed in which the active, torque sensing region is formed directly on the shaft itself, rather than on a separate ferromagnetic element which then has to be affixed to the shaft. See, for example, U.S. Pat. No. 6,047,605 to Garshelis, which is incorporated herein by reference. In one form of these so-called “collarless” transducers, the magnetoelastically active region is polarized in a single circumferential direction and itself possesses sufficient magnetic anisotropy to return the magnetization in the region, following the application of torque to the member, to the single circumferential direction when the applied torque is reduced to zero. The torqued shaft is desirably formed of a polycrystalline material wherein at least 50% of the distribution of local magnetizations lie within a 90-degree quadrant symmetrically disposed around the direction of magnetic polarization and have a coercivity sufficiently high that the transducing region field does not create parasitic magnetic fields in proximate regions of the shaft of sufficient strength to destroy the usefulness, for torque sensing purposes, of the net magnetic field seen by the magnetic field sensor. In particularly preferred forms of such transducers the shaft is formed of a randomly oriented, polycrystalline material having cubic symmetry and the coercivity is greater than 15 Oersted (Oe), desirably greater than 20 Oe and, preferably, greater than 35 Oe. Those characteristics may be achieved in large measure by selecting an appropriate ferromagnetic material for the shaft and active regions.
Because magnetic fields, in the context of their measurement, are fungible, the sensors taught by the above and other prior art may be susceptible to other magnetic fields of external origin. Magnetic fields of external origin are referred to as magnetic noise. In particular, the earth's magnetic field will cause a phenomenon known as “compassing,” in which the measured field is the sum of the torque induced magnetic field and the earth's magnetic field. Within the context of this disclosure, the term “compassing” shall be used to describe any error resulting from the earth's magnetic field.
Magnetic fields of external origin can emanate from both far field and near field sources. A far field source, such as the earth with its magnetic field, generally has the same effect on each magnetic field sensor in a torque sensing device having multiple magnetic field sensors. Near field sources, such as permanent magnets, magnetized wrenches, motors, solenoids, etc., may create magnetic fields having significant local gradients, thus having significantly different effects on the different magnetic field sensors in a torque sensing device having multiple magnetic field sensors. Furthermore, the nearby presence of a ferromagnetic structure may distort the shape and direction of the earth's magnetic field, creating a localized area in which the magnetic flux is concentrated in an undesirable direction. Each of these examples results in a divergent magnetic field, i.e., one in which there are significant local gradients in both magnetic field strength and flux direction.
U.S. Pat. No. 5,520,059 to Garshelis addresses the compassing issue with respect to far field sources. In that patent, a shaft is described having two axially distinct magnetically conditioned regions, equally polarized in opposite circumferential directions. This arrangement yields two torque-dependent magnetic fields and, because the acquiescent magnetizations of the regions are in opposite directions, the torque-dependent magnetic fields are of equal but opposite magnetic polarity. Corresponding with the two regions described in the '059 patent are two magnetic field sensors, each with an opposite axial polarity to the other (but with the same polarity relative to each of the corresponding magnetically conditioned regions). Thus, an ambient magnetic far field affects each of the magnetic field sensors in an equal but opposite manner. Accordingly, by summing the outputs of the magnetic field sensors, measurements resulting from all common mode external magnetic fields, i.e. far fields, are canceled. In applications employing such a scheme, the oppositely polarized sensors should be placed as close to each other as possible to preserve the efficiency of the common mode rejection scheme. Sensors that are spaced from one another exhibit reduced common mode rejection efficiency, as the earth's magnetic field may be significantly distorted around ferromagnetic parts in and around the torque sensor.
While the teachings of the '059 patent are effective when dealing with far fields, a divergent near field can expose each of the two magnetic field sensors to distinctly different field intensities and direction. In this scenario, the two magnetic field sensor outputs will not reflect equal but opposite error components that cancel each other, but rather unequal and opposite components that introduce an error to the measurement. In practice, the configuration of the invention described in the '059 patent may be error-prone in the presence of locally divergent magnetic fields because the two magnetic field sensors experience different magnitudes of the divergent magnetic fields. The difference in magnetic fields between the two magnetic field sensors originating from a near field source combines non-uniformly with torque induced magnetic fields and leads to a false torque value. Thus, it is important to eliminate this near field effect.
There are numerous methods for canceling the effects of near field source or stray magnetic fields. These include employing shielding and using flux directors. Each of these types of structures is made from materials having a high magnetic permeability, meaning that they present a much lower resistance to magnetic fields than, for example, air. In principle, a shield would be in the form of a tube of infinite length, although shorter finite lengths may suitably function. Magnetic fields originating outside of the shield are effectively shunted through the highly permeable shield material, which prevents them from intersecting the field sensors.
While the shielding method noted above can be effective for external magnetic fields perpendicular to the axial direction of a shield in the form of a tube, this shield is very vulnerable to external magnetic fields in the axial direction of the tube which is open at both ends. Any external magnetic fields can transfer to the field sensors inside the shield through the sides of the shield which are open.
Using a different approach, a flux director “gathers” most of the torque dependent magnetic field and directs it into the magnetic field sensors. With this approach, the flux director geometry is such that its effectiveness of gathering the torque dependent magnetic field of interest is much greater than its effectiveness of gathering extraneous and error inducing magnetic fields, thus increasing the efficiency of the magnetic field sensors and hence, their signal to noise ratio.
Flux director structures typically operate by gathering the radial flux component of the torque dependent magnetic field, and are therefore well suited for rejecting axially directed flux of external origin, however, flux directors tend to be susceptible to external fields perpendicular to the axis of the shaft.
A combination of tubular shielding and flux directors would act in a complimentary manner by effectively mitigating both axially and radially directed fields of external origin acting directly on the field sensing devices. Such a combination, however, has other shortcomings that limit its desirability in many applications including cost and packaging within the design.
If an external magnetic field source is directly contacted with the end of a shaft such as the end of the column of an electric power steering system, a strong external near field could transfer to the field sensors through the shaft as a result of diametric variations in the shaft or nearby magnetically coupled structures such as, for example, a bearing or mounting flange. Moreover, a typical manufacturing process for a column or shaft may include a magnetic particle inspection (MPI) process that involves a magnetization process for guiding magnetic particles into the defect sites for visualization of defects on column surface, and a demagnetization process after finishing the inspection. Frequently, demagnetization is not perfect, and there remains a remanant magnetic field in the column or shaft after the MPI process. Typical values of the remanant magnetic fields are between 10 and 100 Gauss. This relatively large external magnetic field can be directly transferred to the field sensors inside the shield, and can be non-uniformly summed with the torque-induced magnetic fields, corrupting the torque measurement. This means that there is no totally effective way to protect or shield external magnetic fields propagating through the shaft with current techniques.
An additional disadvantage of the shielding method is that any deformation of the shield device caused by mechanical impact or extreme temperature change can affect the relative position of the field sensors and the shield, which can lead to unbalancing of far field values between two sensor fields operating in pairs that are oppositely oriented. This would result in compassing failure.
Furthermore, in most torque sensor applications, packaging space is limited, and in many cases there is no room for a shield or flux director. In addition, the added financial cost for those components is not insignificant because materials with high permeability tend to have high percentages of nickel, the pricing of which is quite volatile.
U.S. Pat. App. Pub. No. 2009/0230953 to Lee, which is incorporated herein by reference, describes a torque sensing device designed to cancel near field magnetic noise from external sources without canceling a torque-induced magnetic field. That reference describes a torque sensing device including a shaft having two identically axially polarized primary magnetic field sensors, circumferentially spaced proximate to a magnetically conditioned region on the shaft, the magnetically conditioned region polarized in a circumferential direction. The torque sensing device also includes two secondary magnetic field sensors, each of the secondary sensors evenly spaced from the primary sensors in opposite axial directions, proximate to the shaft, and axially spaced from the magnetically conditioned region such that the secondary sensors are not affected by torque-induced magnetic fields. The secondary sensors are polarized in a direction opposite the direction of polarization of the primary sensors. It is assumed that a near field noise source has its greatest effect on the secondary sensor nearest the source and its least effect to the secondary sensor farthest from the source. It is also assumed that the effect of the noise source on each of the primary sensors is equal to the average of the source's effect on each of the secondary sensors. Thus, by summing the outputs of the two primary and two secondary magnetic field sensors, the effect of the near field noise source is canceled, and the resulting composite signal is representative only of the torque-induced magnetic field.
The configuration of the invention described in the '953 publication can be error-prone in that its effectiveness is based on the assumption that noise-induced magnetic fields decrease linearly as distance from the noise source increases. In practice, however, noise-induced magnetic gradients are typically non-linear. In addition, because the design of the torque sensing device described in the '953 publication requires that the primary sensors be circumferentially spaced, the primary and secondary sensors described in that publication are not axially aligned. Axial alignment of the magnetic field sensors is preferred because such alignment increases the effectiveness of the torque sensing device.
What is needed, therefore, is a torque-sensing device that addresses the issue of torque applied to a rotatable shaft in the presence of a non-linear magnetic noise field gradient, wherein the device does not require additional magnetic shielding elements. What is also needed is a device specifically designed to measure both the torque-induced magnetic field and the noise-induced magnetic field at a time at which torque is applied to the shaft.